High-frequency dielectric heating apparatus



Sept. 14, 1948. B. CASSEN HIGH-FREQUENCY DIELECTRIC HEATING APPARATUSFiled Seyt. 28, 1944 H z'gh Freq ue nay Oscz'llazar WITNESSES:

n Rm M mm m M01 m m d x mu 6 Patented Sept. 14, 1948 HIGH-FREQUENCYDIELECTRIC HEATING APPARATUS Benedict Cassen, Los Angeles, Calif., minorto Westinghouse Electric Corporation, East Pittaburgh, Pa., acorporation of Pennsylvania Application September 28, 1944, Serial No.556,148

4 Claims.

This invention relates to high-frequency dielectric heating of largeslabs or other objects between heating electrode-means having anelectrical length or expanse comparable, in a sense, to the wave lengthof the high-frequency power supply; but in a more particular sense theinvention may be considered to be directed to dielectric heat-treat ntsystems which utilize frequencies high enough in relation to theelectrical characteristics of the power transmitting and consumingelements of the system to ordinarily establish an undesirable standingwave potential pattern along the surface of the heating electrodesbetween which ,the material :being clielectrically heated is disposed,the part of the standing wave on the electrodes having maximum andminimum values separated sufllciently in magnitude to be a factor in theheating of the material.

An important object of my invention is to provide an economical andefllcient dielectric heating system having electrically long heatingelectrodes, in which the material between the electrodes can be heatedthroughout with a satisfactory degree of uniformity.

A further object of my invention is to provide a dielectric heatingsystem of a type described for use in successively heating material inbatches, the system being such as to substantially uniformly heat-treateach batch without requiring adjustments or special attention to theelectrical system.

In dielectric heating, with other factors unchanged or equivalent. theheat generated in a dielectric material between a pair of spacedelectrodes of diflerent high-frequency potentials, in a line directlyacross the material in a direction from one electrode to another,depends on the I square of the voltage impressed across the ends of suchline. With electrodes on which a standing wave pattern exists, thevoltages across difierent pairs of opposite points on the electrodeswill follow the standing wave pattern. If the wave pattern hasrelatively deep depressions, the voltages across the various elements ofthe material at different places between the electrodes will not be thesame. If the system is such that these unequal voltages result also inunequal voltagegradients for the respective elements of the material,the material will not be uniformly heated throughout.

A primary object of my invention is to provide a dielectric system of atype utilizing a power supply of such high frequency as to produce adistinct wave pattern on the heating electrodes, with means tending tocounteract or nullify the 2 variations in the rate of heat generation inthe material which arise out of the diflerent voltages across thediflerent material-portions.

It is known that, under ideal conditions. stand- "ing waves on anelectrically long transmission line which feeds a concentrated load as aterminal impedance, can be eliminated through the use of a stub line, orthe equivalent, which provides an impedance of such value, parallelingthe load, that the resulting terminal impedance, comprising the load andthe stub in parallel, is equal to the characteristic impedance of thetransmission line. For dielectric heating it has been suggested, asdescribed in an article by Bierwirth and Hoyler entitled RF heatingapplied to wood gluing, published on pages 529 to 537 of the October1943, I. R. E., that the tendency for nonuniform dielectric heatingbetween platenelectrodes might be minimized by loading theplaten-electrodesat various points about their periphery with networkswhich, together with the reactance of the platen-electrodes, match thecharacteristic impedance of the line feeding the power to theplaten-electrodes. Such matching, under some conditions, is claimed toeliminate standing waves.

Under practical conditions, the heating load between the electrodesforms a distributed varying impedance, and the standing wave patternalong the platen-electrodes does not remain constant during aheat-treatment cycle for a particular batch of material. One of thereasons for this is the changing electrical properties of the materialas it is heat-treated. Any impedance matching is disturbed. As one batchwhich has been heated is replaced by another to be heated, stilldifferent conditions may be expected. Accordingly, a principal object ofmy invention is to provide a system in which the existence of standingwaves, which may vary because of the changing electrical characteristicsof the material being heated, is accepted, the system, however, beingprovided with separate means tending to equalize the heating of thedifferent parts of the material. This is accomplished by forcing thewave pattern on the heatlng electrodes to continually or continuouslymigrate or shift so that the points of the wave pattern sweep over theelectrodes, with each part i the material subjected to maximum orminimum voltages of the wave pattern for the same time periods. Thus.the average voltage distribution over the electrodes during a heatingperiod tends to be equalized.

Other objects, features and innovations of my invention will bediscernible from the following description and the accompanying drawingwhich diagrammatically illustrates a preferred form of my invention. Inthe drawing:

Figure 1, which is not to scale, is a view, with parts in section, of asimplified dielectric heating system embodying my invention, and r Fig.2 is a curve for explaining the operation of the invention.

Referring to the drawing, a high frequency source of supply I, which maybe a tube-oscillator generator, supplies energy through a transmissionline 2 having .a conductor 4 connected to an edge of a flatplate-electrode 8 and another conductor 8 connected to the flatplateelectrode I which is insulated and spaced from the oppositeplate-electrode 6. In a utilization of my invention, the electrodes 6and I0 may be pressed in a direction toward each other for pressing onopposite sides of a plywood assembly or similar dielectric material. Theplywood comprises the desired number of alternating layers of woodveneer l2 and glue H which are to be bonded under heat and pressure,three such layers being shown in the drawing for simplicity.

An end of a transmission line It is connected to each of the edges ofthe plate-electrodes 8 and i0, opposite to the edges to which thetransmission line 4 is connected, the line I6 being as short asconveniently possible and terminating in a loop l8 coupled to a tunablemetal chamber or resonant cavity 20. The electrical characteristics ofthe chamber 20 can be varied so that the transmission line [6 andchamber 23 comprise a variable impedance network indicated in itsentirety by the reference numeral 22.

The chamber 20 includes a tube 24 concentrically inside a longer andlarger tube 28, with an end of the tube 24 lying in the plane. of an endof the tube 25. The space between these coplanar ends is closed by anannular disc 23; and the other ends of the tubes are closed by an innerclosure 30 and an outer closure 32. An electric motor 34 has a metalshaft 36 conductively passing through the closure 32, the axis of theshaft being parallel to but spaced from, or out of line with, the axesof the concentric tubes 24 and 26. A circular metal disc 33 iseccentrically secured to an end of the shaft 36 in slightly spacedrelation to the closure 33 of the inner tube 24. As the motor 34rotates, the natural resonant frequency of the chamber 20 periodicallychanges so that the impedance of the network 22 is continuously variedin a cyclical manner. Preferably, the resonant value of the tunablechamber 20 corresponding to the frequency of the source 2, is reachedwhen the disc 38 is at some point intermediate its maximum and minimumcapacitive positions with respect to the closure 30. This means that thechamber 20 will present both inductive and capacitive reactance, in thepreferred form, but at different times.

In dielectric heating, high frequencies in the order of megacycles aredesirable for faster heating with practicable voltages. Frequencies inthe order of 1.5450 megacycles and much more are satisfactory ordesirable. With air as a dielectric, a standing sinusoidal voltage wavehas a wave length of meters at a frequency of 30 megacycles. Thedistance between the maximum or minimum scalar values of such a wave isa quarter wave length or 2.5 meters. This corresponds roughly to 8 feet.The length of this quarter wave decreases in proportion to .but greaterthan the minimum.

the square root of the dielectric constant of the medium through whichthe incident and reflected waves, which establish the standing waves,are propagated. In a material with a dielectric constant of 3, thespace-separation between the maximum and minimum magnitudes of the wavewill be about 4.6 feet for the case mentioned. Heating electrodes about9 feet in length and much more are usually desirable in making plywood,the spacing between electrodes being generally relatively much less.

Fig. 2 shows several positions of a wave pattern as it is shiftedlongitudinally along the electrodes having a length represented by thedistance AB, the point A corresponding to the point at which thetransmission line 2 is connected. If the shift caused by a singlerevolution of the disc 33 is more than wave length, the change involtage at any point on the electrodes will include the maximum andminimum values of the standing wave. Cyclical variations in potentialare produced by continuous rotation of the disc 33 on the assumptionthat the shape of the standing wave does not alter. The power absorbedby the dielectric material is a function of the second power of thevoltage so that the minimum value is that whq'e the curve crosses theabscissae axis shown. The curve C indicates the standing wave pattern onthe elec trodes at a time when the voltage through the material on aline x distance from point A, perpendicular to the electrodes, is amaximum. As the disc 38 rotates, the wave pattern moves, and the curve Dwill represent the pattern at some other instant. The voltage at X atthis time will be considerably less than the maximum At some still laterinterval, the voltage at X has passed through the minimum value andbecomes greater but in the opposite polarity-sense, as indicated by thecurve E.

If the curves C and E represent the maximum shift of the wave patternduring a revolution of the disc 33, it is evident that the potential atevery point on the electrodes will cyclically vary in accordance withthe shape of the standing wave, and in order to get the same variationat each point, the shift caused by rotation of the motor 34 should alsobe at least a half of a wave length. However, it is obvious that lengthswhich are substantially multiples of a half of a wave length areutilizable for the same purpose.

The speed of the motor may be varied through a wide range, but thestanding wave should sweep back and forth across the electrodes at leastseveral times through a heat-treatment period for the material. However,a comparatively rapid sweep is desirable in order to keep the heatingmore uniform and to minimize the effects of heat radiation andconduction, and the changes introduced by the curing or setting of thematerial. Since the heat-treatment period may require from a few minutesto a. few hours, a motor speed of 30 revolutions per second causes anadequate shift of the wave pattern back and forth across the electrodesthrough any of the aforesaid heating periods.

In narrow relatively long rectangular heating electrodes, the standingwave pattern can be considered as producing substantially the samepotentials on a line across the narrow width of the electrodes. Wherethe electrodes have a greater width, which also causes a variation inpotential therealong, it may be desirable to connect one or morenetworks 22 at several different points around the edges of theelectrodes.

While I have described my invention in connection with a preferred formand operation thereof, it is obvious that other forms and networks canbe used embodying the teachings and principles herein disclosed. Anothersystem and a method for obtaining more uniform heating by forciblychanging the wave-patterns are disclosed and claimed in a copendingapplication of Robert M. Baker, Serial No. 556,140, filed concurrentlyherewith.

I claim as my invention:

1. Dielectric heating apparatus comprising a pair of relatively largespaced electrodes for heating a dielectric material therebetween, asource of high-frequency electrical energy adapted to establish anon-flat wave pattern along said electrodes, a transmission lineconnecting said source of energy to said electrodes, a variableterminating impedance connected to said electrodes at a point removedfrom said transmission line, and means for automatically cyclicallychanging said impedance.

2. Dielectric heating apparatus comprising a pair of relatively largespaced electrodes for heating a dielectric material therebetween, asource of high-frequency electrical energy adapted to establish anon-flat wave pattern along said electrodes, a transmission lineconnecting said source of energy to said electrodes, a resonant chamberconnected to said electrodes at a point removed from said transmissionline, and means for cyclically continually changing the electricalcharacteristics of said chamber during a heating operation.

3. Dielectric heating apparatus comprising a pair of relatively largespaced electrodes for heating a dielectric material therebetween, asource of high frequency electrical energy, means connecting said sourceof energy to said spaced electrodes, a variable impedance network meansalso connected to said electrodes, said source of 5 Number saidconnections including a pair of spaced heating electrodes for receivingdielectric material therebetween for a heat-treatment period, the systembeing such as to provide a wave-pattern on said electrodes, thewave-pattern having spaced maximum and minimum points, and separatemeans associated with said network operable for automatically varyingsaid network in a predetermined manner for sweeping the wave-patternback and forth over the electrodes, during a heat-treatment period,whereby to provide substantially the same average heating throughoutsaid material.

BENEDICT CASSEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Davis Aug. 28, 1934 Chaifee Feb. 21,1939 Keller Nov. 7, 1939 Blewett et a1. May 13, 1941 Alford June 10,1941 Evans Dec. 1, 1942 Rouy Dec. 15, 1942 Bierwirth Jan. 1:, 194a

